The invention relates to a demultiplexer for a combined color television signal.
It is known that, in order to prevent large scale flickering of television images the image frequency, or frame rate, should be raised to 100 Hz, or 120 Hz, from the present 50 Hz, or 60 Hz, respectively. In order to keep the cost of the storage elements for digitalized image data required to temporarily store the digitalized image data within limits, the scanning rate has been fixed at 10.125 Mhz for luminance components and at 3.375 Mhz for chrominance components with line-jump scanning.
These frequencies are high enough for natural images, but are too low for high resolution images such as computer graphics In the latter case, higher scan frequencies are preferred, in particular; 13.5 MHz for luminance and 6.75 MHz for chrominance, whereby the number of image points is increased in every line. Thus, the total scanning rate is doubled again, at least for the chrominance components, when compared to the flicker-free line-jump scanning.
From the above discussion it follows that the scanning rate for luminance is 13.5 MHz, for the U component of the chrominance it is 6.75 MHz and for the V component of the chrominance it is also 6.75 MHz. The total scanning rate for chrominance is thus also 13.5 MHz.
Presently available image memories are not able to store this quantity of data. In order to nonetheless obtain a higher image frequency, the individual lines would have to be repeated The disadvantage of the required switching arrangement is that a line memory would need to be installed in addition to the image memory.